Process for preparing a group V111-metal containing catalyst, use thereof for preparing an alkenyl carboxylate

ABSTRACT

A process for preparing a catalyst by
         (a) selecting a carrier which is a silica based carrier which has been subjected to a series of washings with one or more aqueous liquids consisting of aqueous liquids which have a pH of least 3, when measured at 20° C., or which is a silica based carrier which is formed from materials one or more of which have been subjected to this series of washings,   (b) precipitating a Group 8 metal compound onto the carrier,   (c) converting the precipitated Group 8 metal compound into metallic species, and   (d) subjecting the Group 8 metal/carrier composition to a purification treatment, before or after step (c); a catalyst which is obtainable by this process; and a process for preparing an alkenyl carboxylate by reacting a mixture comprising an olefin, a carboxylic acid and oxygen in the presence of the catalyst.

FIELD OF THE INVENTION

The invention relates to a process for preparing a catalyst, to acatalyst which is obtainable by the process of this invention, and to aprocess for preparing an alkenyl carboxylate comprising reacting amixture comprising an olefin, a carboxylic acid and oxygen in thepresence of the catalyst.

BACKGROUND OF THE INVENTION

Catalysts for the preparation of an alkenyl carboxylate from an olefin,a carboxylic acid and oxygen are known in the art. Such catalysts arebased on a Group 8 metal as a catalytically active metallic species on acarrier. The preparation of the catalysts is well documented.

For example, the process for preparing the catalyst of U.S. Pat. No.4,048,096 comprises the steps of selecting a carrier, precipitating aGroup 8 metal compound into the carrier, converting the precipitatedGroup 8 metal compound into metallic species, and subsequently purifyingthe catalyst by washing with water.

Similar schemes for the preparation of the catalysts are known from U.S.Pat. Nos. 5,179,057 and 5,189,004. The latter documents highlight theremoval from the catalysts of sodium ions, which are introduced duringthe catalyst preparation, for example as a portion of a precursor of theprecipitated Group 8 metal compound (for example sodiumtetrachloropalladium (II)), or as a portion of the precipitating agentwhich is used for precipitating the Group 8 metal compound (for examplea sodium silicate or sodium hydroxide). Both documents teach that theremoval of sodium ions leads to an increase in the activity of thecatalyst and to a decrease in the selectivity when the catalyst is usedin the process for the preparation of an alkenyl carboxylate.

U.S. Pat. Nos. 5,250,487 and 5,422,329 relate to Group 8 metal catalystsfor use in a process for the preparation of an alkenyl carboxylate froman olefin, a carboxylic acid and oxygen. The catalysts are based oncarrier particles which have been pressed with the aid of a binder ofone or more salts of carboxylic acids. The carrier particles are washedwith an acid for the removal of the cations of the binder from thesupport particles. The acid may be a mineral acid such as hydrochloricacid, sulfuric acid, phosphoric acid, or nitric acid. If the anion ofthe acid is detrimental to the catalyst, such as chloride and sulfateanions, excess acid may be removed by washing out with distilled water.If the salts employed during the subsequent catalyst preparation containconstituents harmful to the catalyst, such as chloride or sulfate, thecatalyst is washed with water.

Thus, in the prior art documents relating to the Group 8 metal catalystsattention has been paid to the detrimental effects of certain catalystimpurities which are introduced intentionally during the carrier orcatalyst preparation and to the removal of these impurities. Suchimpurities may be removed by using dedicated means. For example, thecations of the binder are removed by washing with acid.

WO-00/15333 teaches the preparation of improved catalysts, in particularsilver impregnated alumina based catalysts for the vapor phaseproduction of epoxides. The improvement is achieved by lowering theconcentration of ionizable species present on the surface of thecarrier, in particular by treating the carrier by washing with boilingwater prior to catalyst preparation.

Although the known Group 8 metal catalyst have appreciable activity andselectivity in the preparation of alkenyl carboxylate from an olefin, acarboxylic acid and oxygen, further improvements of these catalysts aredesirable, in particular in their ability to maintain their level ofactivity and selectivity during use over a long period of time.

SUMMARY OF THE INVENTION

It has now unexpectedly been found that the Group 8 metal catalystshaving an improved catalyst performance in the preparation of alkenylcarboxylates are prepared if prior to the catalyst preparation thecarrier is subjected to washing with water, even though water washing isalso applied in the course of the catalyst preparation.

By the term “improved catalyst performance” it is meant that there is animprovement in at least one of the catalyst properties, which catalystproperties include catalyst activity, selectivity, activity orselectivity performance over time, operability (i.e. resistance torun-away), conversion and work rate. By “selectivity” it is meant theselectivity to alkenyl carboxylate, based on the quantity of olefinconverted. The improvement in question concerns in particular theability of the catalyst to maintain its level of activity andselectivity during use over a long period of time.

This result is unexpected in view of the prior teachings which relate tothe Group 8 metal catalysts, as referred to hereinbefore. Namely, oneskilled in the art would expect that the washings in the course of thecatalyst preparation, as taught by these documents, would lead also tothe removal of impurities already present in the carrier per se, so thatno further advantageous effect could be expected from an additionalwashing of the carrier, i.e. prior to the catalyst preparation.

The beneficial effect of subjecting the carrier to washing with water,in addition to washing in the course of the catalyst preparation is alsounexpected in view of WO-00/15333, referred to hereinbefore. Namely,WO-00/15333 does not teach any subsequent washing, i.e. as a step of thecatalyst preparation or subsequent to the catalyst preparation.

Accordingly, the invention provides a process for preparing a catalystwhich process comprises the steps of

(a) selecting a carrier which is a silica based carrier which has beensubjected to a series of washings with one or more aqueous liquidsconsisting of aqueous liquids which have a pH of least 3, when measuredat 20° C., or which is a silica based carrier which is formed frommaterials one or more of which have been subjected to a said series ofwashings,

(b) precipitating a Group 8 metal compound onto the carrier,

(c) converting the precipitated Group 8 metal compound into metallicspecies, and

(d) subjecting the Group 8 metal/carrier composition to a purificationtreatment, before or after step (c).

The invention also provides a process for preparing a catalyst whichprocess comprises the steps of

(a) washing a carrier with one or more aqueous liquids consisting ofaqueous liquids which have a pH of least 3, when measured at 20° C.,wherein the carrier is a silica based carrier,

(b) precipitating a Group 8 metal compound onto the carrier,

(c) converting the precipitated Group 8 metal compound into metallicspecies, and

(d) subjecting the Group 8 metal/carrier composition to a purificationtreatment, before or after step (c).

The invention also provides a catalyst which is obtainable by a processfor preparing a catalyst according to this invention.

The invention also provides a process for preparing an alkenylcarboxylate comprising reacting a mixture comprising an olefin, acarboxylic acid and oxygen in the presence of the catalyst of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

The silica based carrier for use in this invention may be of any kind.For example, the carrier may comprise further materials such as alumina,magnesia, zirconia, fuller's earth, artificial and natural zeolites, andcombinations thereof, in particular alumina. The silica content of thecarrier is typically at least 50% w, more typically at least 90% w,based on the weight of the carrier. Frequently the silica content of thecarrier is at most 99.99% w, more frequently at most 99.9% w, on thesame basis.

Typically, the carrier is a porous carrier, preferably having a specificsurface area of at least 0.01 m²/g, in particular in the range of from0.05 to 1000 m²/g, more in particular in the range of from 0.2 to 1000m²/g, as measured by the B.E.T. method, and a water absorption capacityof from 0.05 to 3 ml/g, in particular from 0.1 to 2 ml/g, as measured byconventional water absorption technique. The B.E.T. method as referredto herein has been described in detail in S. Brunauer, P. Y. Emmett andE. Teller, J. Am. Chem. Soc. 60, 309-16 (1938).

Of particular interest are silicas which have a specific surface area inthe range of from 10 to 1000 m²/g, in particular from 50 to 500 m²/g, asmeasured by the B.E.T. method.

Regardless of the carrier used, it may be shaped into particles, chunks,pieces, and the like. Preferably, for use in a tubular fixed bedreactor, they are formed into a rounded shape, for example in the formof spheres, pellets, cylinders, rings or tablets, typically havingdimensions in the range of from 2 mm to 2 cm.

For use in this invention, the carrier is subjected to a series ofwashings with one or more aqueous liquids. A series of washings isherein understood to include a single washing step and a combination ofconsecutive washing steps which employ one or more washing liquids. Inaccordance with this invention the washing liquids comprise aqueousliquids which all have a pH of at least 3, when measured at 20° C. Oneskilled in the art will appreciate that aqueous liquids may contain asmall quantity of acid, such as resulting from dissolution ofatmospheric carbon dioxide, and they may therefore have a pH slightlybelow 7, for example down to a pH of 3. Such aqueous liquids containvery little acid or the acid is a weak acid, so that yet they areconsidered to be essentially non-acidic aqueous liquids.

Preferably, the aqueous liquids have all a pH of at least 5, inparticular at least 6, more in particular at least 7, when measured at20° C. Typically, the washing liquids have all a pH of at most 10, inparticular at most 9, more in particular at most 8, when measured at 20°C.

As used herein, the term “pH” refers to the pH of an aqueous liquid asmeasured by using a conventional pH measuring probe which is calibratedby using buffer solutions.

Eligibly, the aqueous liquids comprise for the greater part water, andthey may or may not comprise relatively small quantities of othercomponents, for example organic materials, for example esters, ethers,alcohols or ketones, or salts like acetates, carbonates, nitrates oroxalates, in particular such salts as lithium, sodium, potassium,ammonium, monoalkylammonium, dialkylammonium, trialkyl-ammonium andtetraalkylammonium salts. Such other components may not be detrimentalto the preparation of the catalyst or to the performance of the catalystwhen used in the preparation of alkenyl carboxylates. Otherwise, suchother components, when left behind on the carrier after the washing maybe removed from the carrier, for example by further washing the carrier,by evaporation or by decomposition (i.e. by calcination).

Not wishing to be bound by theory it is believed that as a result of thewashing ionizable species are removed from the carrier, or at least fromthe carrier surface, which ionizable species have an influence on theprecipitation and/or the conversion into metallic species, such that themorphology of the active surface of the catalyst is changed to an extentwhich favors the catalyst performance in the preparation of alkenylcarboxylates. It is believed that the following ionizable species may beassociated with these effects: silicates, aluminosilicates, sulfates,chlorides, sodium salts, aluminium salts, calcium salts, magnesiumsalts, and the like.

The aqueous liquids which comprise a salt as specified hereinbefore maybe called ion exchange solutions. The presence of salts in the aqueousliquids may facilitate the removal of ionizable species which are firmlybound to the carrier, so that a desirable result may be achieved in ashorter time or at a lower temperature.

Suitably, the ion exchange solution comprises the salt in a quantity ofat most 0.1 moles/l. Suitably, the ion exchange solution comprises thesalt in a quantity of at least 0.001 moles/l. Preferably, the ionexchange solution comprises the salt in a quantity in the range of from0.002 to 0.05 moles/l. The remainder of the solution may be a de-ionizedliquid as specified hereinafter.

The water content of the aqueous liquids, in particular when they do notcomprise added salt as to form an ion exchange solution, is preferablyat least 90% w, more preferably at least 99% w, in particular at least99.9% w, more in particular at least 99.99% w, relative to the weight ofthe aqueous liquid. Frequently, the content of water is at most 99.999%w, on the same basis. Preferably, the aqueous liquid is water.

Suitably, the aqueous liquids have a low conductivity. Suchlow-conductivity aqueous liquids typically do not comprise added salt asto form an ion exchange solution. Suitably, the conductivity is at most500 μmho (mho is Ω⁻¹, or Siemen, or S), more suitably at most 100 μmho,preferably at most 20 μmho, in particular at most 5 μmho, when measuredat 98° C. Frequently the conductivity will be at least 0.1 μmho, morefrequently at least 0.2 μmho, on the same basis. Conductivities areherein understood to be electrical conductivities, measured by using aconductivity measuring probe having a cell constant of 1.0/cm. Suitablya YSI Model 3401 (trademark) conductivity measuring probe is used,connected to a YSI Model 35 (trade mark) conductance meter. Suchlow-conductivity aqueous liquids are typically de-ionized aqueousliquids. The de-ionized aqueous liquids are obtainable by de-ionisationusing an ion exchange material such as an ion exchange resin, typicallya cation exchange material in the acidic form, or an anion exchangematerial in the basis form, but preferably using a cation exchangematerial in the acidic (H⁺) form and an anion exchange material in thebasic (OH⁻) form.

The extent and the type of washing are not material to the presentinvention. The washing may be carried out in a continuous fashion or itmay be a batch type operation. There may be one washing, but the numberof washings may also be two, or three, or more, for example up to fiveor ten. The quantity of aqueous liquid used in the washings relative tothe quantity of the carrier is also not material to the invention. Thewashing may be carried out a temperature in the range of from 10 to 300°C., preferably at a temperature in the range of from 50 to 150° C., forexample about 100° C. However, when using an ion exchange solution, thetemperature is preferably in the range of from 20 to 120° C., forexample about 70° C.

The washing may be monitored by applying a conductivity test, whichconductivity test involves contacting samples of water with samples ofthe carrier, unwashed and washed, and measuring the conductivity of eachwater sample after it has reached equilibrium with the respectivecarrier sample at 95° C. In this conductivity test the conductivity ismeasured at 95° C., the quantity of water sample is 3 g/g carrier sampleand the conductivity of the water prior to the contacting with thecarrier sample is 1.5 μmho at 98° C. Water eligible for use in theconductivity test is water which is de-ionized using a cation exchangematerial in the H⁺ form and an anion exchange material in the OH⁻ form.

Typically the carrier is washed to the extent that in the conductivitytest the conductivity measured for the washed carrier is less than 50%of the value found for the unwashed carrier, preferably less than 30%,more preferably less than 20%. Frequently, the carrier is washed to theextent that in the conductivity test the measured conductivity is atleast 1%, more frequently at least 5% of the value found for theunwashed carrier.

Typically the carrier is washed to the extent that in the conductivitytest the conductivity measured for the washed carrier is less than 200μmho, more typically less than 75 μmho, preferably less than 50 μmho.The carrier may be washed such as to achieve that the conductivitymeasured in the conductivity test is as low as possible. However, inpractice the carrier may be washed to the extent that the conductivitymeasured in the conductivity test is above 2 μmho, more frequently above3 μmho.

As an alternative to, or in addition to washing the carrier, one or morematerials from which the carrier is formed may be subjected to a seriesof washing as, and to the extent described hereinbefore. Subsequently,the carrier may be formed from the materials by conventional mixingand/or shaping methods, such as extrusion.

The Group 8 metal for use in this invention has suitably an atomicnumber of at least 44 and at most 78. One or more Group 8 metals may beapplied. Preferably, the Group 8 metal is palladium.

Preferably, the catalyst is based on a Group 1b metal, in addition to aGroup 8 metal. One or more Group 1b metals may be applied. The Group 1bmetal is preferably gold.

The terms “Group 8 metal” and “Group 1b metal” as used herein refer tothe metals of Group 8 and Group 1b, respectively, of the Periodic Tableof the Elements as published in R C Weast (Ed,) “Handbook of Chemistryand Physics”, 54^(th) edition, CRC Press, inside cover.

In a preferred embodiment, the catalyst is based on palladium as theGroup 8 metal and gold as the Group 1b metal.

The term “Group 8 metal/carrier composition”, as used herein refers toany composition comprising the carrier and a Group 8 metal dispersed onthe carrier, irrespective of whether the Group 8 metal is present as aGroup 8 metal compound or Group 8 metal compound precursor, or in theform of metallic species.

Suitably the Group 8 metal compound and optionally the Group 1b metalcompound is precipitated onto the carrier by pore impregnating thecarrier with one or more aqueous solutions comprising a Group 8 metalcompound precursor and optionally a Group 1b metal compound precursorand then precipitating the Group 8 metal compound and optionally theGroup 1b metal compound onto the carrier from such solutions, by using aprecipitating agent. In more detail, the applicable materials andmethods may be those as disclosed in U.S. Pat. Nos. 4,048,096, 5,179,057and 5,189,004, which are incorporated herein by reference.

The volume of an impregnation solution preferably corresponds to atleast 80%, preferably 95 to 100% of the water absorption capacity of thecarrier.

Eligible Group 8 metal compound precursors and Group 1b metal compoundprecursors are for example water soluble acids and salts, such aschlorides, nitrates, nitrites and sulfates. Preferred such Group 8containing acids and salts are palladium (II) chloride, palladium (II)nitrate, and palladium (II) sulfate and, in particular, sodium palladium(II) tetrachloride. Preferred such Group 1b metal containing acids andsalts are auric (III) chloride and, in particular, tetrachloroauric(III) acid.

The precipitating agent includes for example alkali metal hydroxides,alkali metal bicarbonates, alkali metal carbonates and, preferably,alkali metal silicates. Suitable alkali metals are lithium, sodium andpotassium. The preferred precipitating agent is sodium silicate. Auseful form of sodium silicate is sodium metasilicate pentahydrate. Theprecipitating agent is suitably used in an excess relative to the Group8 metal and optionally the Group 1b metal, taken together. For example,the precipitating agent may be used in a quantity of 1 to 3 moles,preferably 1.5 to 2.5 moles per mole of Group 8 metal compoundprecursor. If the Group 1b metal precursor is present an additionalquantity of the precipitating agent may be used, for example 2 to 10moles, preferably 2.5 to 8 moles per mole of Group 1b metal compoundprecursor.

The precipitating agent is preferably used as an aqueous solution. Theaqueous solution has typically a volume sufficient to cover theimpregnated, wet carrier particles. As an alternative, after theimpregnation with one or more solutions comprising a Group 8 metalcompound precursor and optionally a Group 1b metal compound precursor,the carrier particles may be dried and subsequently impregnated with anaqueous solution comprising the precipitating agent. In the latter case,the volume of the aqueous solution of the precipitating agent typicallycorresponds to at least 80%, preferably 95 to 100% of the waterabsorption capacity of the carrier.

The temperature at which the precipitation may be carried out istypically in the range of from 1 to 100° C., more typically in the rangeof from 5 to 50° C., for example about 20° C. The reaction time appliedin the precipitation step may be for example at least 2 hours, morepreferably at least 3 hours, and it may be for example up to 100 hours,more typically it is in the range of from 6 to 40 hours, for example 24hours. During the precipitation, the particles may be left static, orthey may be moved relative to the solution of the precipitation agent,or relative to each other. For example, the particles may be movedrelatively to each other during the initial stages of the precipitation,for example during the first 15 minutes, or first 30 minutes, or firsthour. Upon completion of the precipitation, the pH of the precipitatingsolution is preferably in the range of from 6.5 to 11, for example 6.5to 9.5, but more preferably it is in the range of from 7.5 to 10, inparticular from 7.5 to 8, when measured at 20° C. The final pH may beadjusted by changing the amount of the precipitating agent.

The quantity of the Group 8 metal compound precursor may be such that inthe catalyst as prepared the content of the Group 8 metal is typicallyin the range of from 10 to 500 mmoles/kg catalyst, and preferably in therange of from 20 to 200 mmoles/kg catalyst, for example about 75mmoles/kg or about 138 mmoles/kg.

The quantity of the Group 1b metal compound precursor may be such thatin the catalyst as prepared the content of the Group 1b metal istypically in the range of from 1 to 200 mmoles/kg catalyst, andpreferably in the range of from 5 to 100 mmoles/kg catalyst, for exampleabout 37.2 mmoles/kg or about 65 mmoles/kg.

The precipitated Group 8 metal compound and, if present, the Group 1bmetal compound may be converted into metallic species. If the Group 8metal in the Group 8 metal compound and, if present, the Group 1b metalin the Group 1b metal compound are not in their zero valence state, theconversion into metallic species may be accomplished by reduction.Suitable reducing agent and reduction methods are known from U.S. Pat.Nos. 4,048,096, 5,179,057 and 5,189,004, which are incorporated hereinby reference.

For example, the reduction may be accomplished by using as a reducingagent diborane; amines, such as ammonia and hydrazine; carboxylic acidsand their salts, such as oxalic acid, potassium oxalate, formic acid,potassium formate, ammonium citrate; aldehydes, such as formaldehyde,acetaldehyde; hydrogen peroxide; reducing sugars such as glucose;alcohols other than reducing sugars, such as methanol and ethanol;polyhydric phenols, such as hydroquinone and catechol; hydrogen; carbonmonoxide; olefins, such as ethylene, propene and isobutene; or sodiumborohydride. The reduction may be carried out in an aqueous solutionwhich comprises for example at most 50 mole, preferably at most 25 moleof the reducing agent per mole of Group 8 metal present in the Group 8metal compound and, if present, the Group 1b metal in the Group 1b metalcompound. This will frequently result in a complete reduction of theGroup 8 metal and, if present, the Group 1b metal. Suitably, so much ofthe reducing agent is used which is sufficient to complete the reductionof at least 90%, more suitably at least 99% of the Group 8 metal and, ifpresent, the Group 1b metal to metallic species.

Preferably, hydrogen is employed as the reducing agent. When hydrogen isemployed, typically no liquid diluent is present, typically the absolutepressure is in the range of from 50 to 2000 kPa (0.5 to 20 bar), moretypically from 100 to 1000 kPa (1 to 10 bar), typically the hydrogenpartial pressure is in the range of from 1 to 2000 kPa (0.01 to 20 bar),and typically the temperature is in the range of from 10 to 300° C.,more typically from 50 to 250° C.

In another preferred embodiment, hydrazine is employed as the reducingagent. When hydrazine is employed, typically an aqueous diluent ispresent, and typically the temperature is in the range of from 0 to 100°C., more typically from 5 to 50° C., for example 20° C.

Combinations of reducing agents may be used, or two or more separatereduction steps may be applied. For example, the precipitated Group 8metal compound and, if present, the precipitated Group 1b metal compoundmay be reduced in part with a first reducing agent, for example about20%-mole or about 40%-mole or about 60%-mole of the total of Group 8metal and Group 1b metal (if any), and in a subsequent step, theremaining part may be reduced with a second reducing agent. The firstand second reducing agents may independently be selected from thereducing agents described hereinbefore.

Preferred first reducing agents are selected from diborane; amines, suchas ammonia and hydrazine; carboxylic acids and their salts, such asoxalic acid, potassium oxalate, formic acid, potassium formate, ammoniumcitrate; aldehydes, such as formaldehyde, acetaldehyde; hydrogenperoxide; reducing sugars such as glucose; polyhydric phenols, such ashydroquinone and catechol; or sodium borohydride. The preferred firstreducing agent is hydrazine.

The second reducing agents may suitably be selected from hydrogen;carbon monoxide; alcohols, such as methanol and ethanol; aldehydes, suchas formaldehyde and acetaldehyde; and olefins, such as ethylene, propeneand isobutene. The preferred second reducing agent is hydrogen.

The reduction in which the first reducing agent is employed ispreferably carried out as a liquid phase reaction, i.e. a reductionwhich involves contacting the Group 8 metal/carrier composition with thefirst reducing agent present in a liquid phase. The reduction in whichthe second reducing agent is employed is preferably carried out as a gasphase reaction, i.e. a reduction which involves contacting the Group 8metal/carrier composition with the second reducing agent present in agas phase, in the absence of a continuous liquid phase, preferably inthe absence of a liquid phase.

In a particular embodiment, a major amount of the precipitated Group 8metal compound and, if present, the precipitated Group 1b metal compoundmay be reduced in the liquid phase reaction by using the first reducingagent. This may be achieved by employing a suitable amount of the firstreducing agent and allowing the first reducing agent to completely reactaway. More of the first reducing agent may be needed if it has atendency to decompose, under the prevailing reaction conditions. Themajor amount may be at least 50%-mole, and preferably it may representfrom 70 to 99%-mole, more preferably from 80 to 90%-mole, of the totalof the Group 8 metal and Group 1b metal (if any). In this particularembodiment, the reduction with the first reducing agent may be followedby the purification step (c), subsequently optionally by a drying stepas described hereinafter, and thereafter the remaining part of the Group8 metal and Group 1b metal (if any) may be reduced with the secondreducing agent, in particular, hydrogen. This sequence of steps isadvantageous as problems are avoided which are associated with thedisposal of excess, unconverted reducing agents (these may be noxiousmaterials, such as hydrazine), while it minimizes possible losses ofGroup 8 metal and Group 1b metal (if present) during the purificationstep (c), and it leads to a high yield of metallic species on thecatalyst. Further, it has unexpectedly been found that by applying thecombination of a liquid phase reaction and a gas phase reaction insteadof applying the liquid phase reaction only, there is an improvement inthe dispersity of the metallic species in the catalyst, as measured byan increased carbon monoxide chemisorption of the catalyst. It istheorized that a better dispersity of the metallic species leads to animproved catalyst performance.

In another embodiment the Group 8 metal compound precursor and, ifpresent, optionally the Group 1b metal compound precursor isprecipitated and converted into metallic species in one step, followingfor example procedures as disclosed in WO-99/08790 and WO-99/08791. Thismeans that in the catalyst preparation according to this invention step(b) and step (c) may be carried out as a single step.

The purification treatment suitably involves a series of washings of theGroup 8 metal/carrier composition, with one or more aqueous liquids,with the objective of removing at least a portion of the uselesschemicals, which are left on the Group 8 metal/carrier composition afteraccomplishing the precipitation of the Group 8 metal compound on thecarrier, and, optionally, after converting the Group 8 metal compoundinto metallic species.

Eligibly, the aqueous liquids for use in the purification treatment maybe selected from the aqueous liquids as specified hereinbefore for thewashing of the carrier of steps (a). The purification treatment may becarried out at a temperature in the range of from 0 to 100° C.,preferably at a temperature in the range of from 5 to 50° C., forexample about 20° C.

The purification treatment may be monitored by any suitable means, forexample by applying the conductivity test as described hereinbefore.Alternatively, the purification treatment may be monitored by followingthe disappearance of contaminants which are to be removed, such assodium ions or chloride, in accordance with the nature of, for example,the Group 8 metal compound precursor, the precipitating agent and thereducing agent. In this respect, reference may be made to U.S. Pat. Nos.4,048,096, 5,179,057 and 5,189,004, which are incorporated herein byreference.

The purification treatment, i.e. step (d), may be carried out after step(c) and before an optional step (e), as described hereinafter, or afterstep (b) and before step (c). As set out hereinbefore, it may beadvantageous to carry out a step (c) before step (d) and to carry outanother step (c) after step (d), and suitably before an optional step(e).

The present process for preparing a catalyst may comprise in additionthe step of (e) impregnating with a source of an alkali metal, such asdisclosed in U.S. Pat. Nos. 4,048,096, 5,179,057 and 5,189,004, whichare herein incorporated by reference. Any source of an alkali metal maybe used such that the alkali metal deposited on the Group 8metal/carrier composition can form an alkali metal carboxylate ormaintain the presence of an alkali metal carboxylate upon contact withan carboxylic acid, such as during the subsequent use of the catalyst inthe preparation of an alkenyl carboxylate.

Suitable sources of an alkali metal are for example alkali metalcarbonates and, preferably, alkali metal carboxylates. The alkali metalcarboxylate is typically derived from a mono carboxylic acid, such asbutyric acid, propionic acid and, preferably, acetic acid. The alkalimetal may be any one or more of lithium, sodium, potassium, rubidium andcesium. Preferably, the alkali metal is potassium. The preferred alkalimetal carboxylate is potassium acetate. The quantity of the alkali metalcarboxylate is typically such that the alkali metal content of thecatalyst is in the range of from 0.1 to 5 mole/kg, more preferably from0.2 to 2 mole/kg catalyst, for example 340 mmole/kg, or 585 mmole/kg, or765 mmole/kg, or 1560 mmole/kg.

The step of impregnating with an alkali metal carboxylate may be carriedout at any stage of the catalyst preparation. Preferably, the Group 8metal/carrier composition is impregnating with an alkali metalcarboxylate after step (d), in particular after step (d) and step (c).

At certain stages of the catalyst preparation it may be desirable toperform a drying step. Drying is typically performed at a temperature inthe range of from 50 to 300° C., more typically in the range of from 80to 150° C., for example 90° C., or 115° C., or 120° C., using an inertgas, such as nitrogen or helium, or air.

A drying step may be carried out, for example, following the initialcarrier washing (cf. steps (a)), following the purification treatment(cf. step (d)) or following the impregnation of step (e). Preferably,the last step in the catalyst preparation in which liquids are involvedis a drying step, after which the catalyst may or may not be subjectedto essentially dry operations, such as milling and sieving.

The catalyst which is prepared by the methods described hereinbefore istypically a shell type catalyst, i.e. a catalyst which comprises thecatalytically active species, i.e. the Group 8 metallic species, in thesurface layer of the carrier. For example, 90 mole-% of the Group 8metallic species may be distributed within the surface layer extendingat most 2 mm from the surface of the carrier. In more preferredembodiments, 90 mole-% of the Group 8 metallic species may bedistributed within the surface layer extending at most 1.5 mm, inparticular at most 1 mm, from the surface of the carrier. Frequently thesurface layer in question extends at least 0.05 mm, in particular atleast 0.1 mm, from the surface of the carrier.

The present process for preparing an alkenyl carboxylate comprisesreacting a mixture comprising an olefin, a carboxylic acid and oxygen inthe presence of the catalyst of this invention. The process isfrequently a gas phase process, wherein a gaseous feed comprising thereactants is contacted with the solid catalyst. The catalyst is suitablypresent in the form of a fluidized bed of catalyst particles, or, moresuitable, in the form of a packed bed. The process may be carried out asa batch process, however, it is more suitable to carry out the processas a continuous process.

The carboxylic acid is preferably a monocarboxylic acid, for examplebutyric acid, propionic acid, or preferably acetic acid.

The olefin is typically a monoolefin, for example 1-butene, 2-butene,isobutene, propylene, or preferably ethylene.

Most preferably, the carboxylic acid is acetic acid and the olefin isethylene, in which case the alkenyl carboxylate is vinyl acetate.

The quantity of carboxylic acid is suitably in the range of from 1 to20%-mole, more suitably in the range of from 5 to 15%-mole, relative tothe number of moles of the feed. The quantity of olefin is suitably inthe range of from 10 to 80%-mole, more suitably in the range of from 30to 60%-mole, relative to the number of moles of the feed. The quantityof oxygen is suitably in the range of from 1 to 15%-mole, more suitablyin the range of from 5 to 10%-mole, relative to the number of moles ofthe feed. One skilled in the art will understand that for a gaseous feeda mole fraction corresponds with a volume fraction.

The source of oxygen may be air. Air may be used in the process of thisinvention, but it is preferred that an oxygen containing gas which maybe obtained by separation from air is used.

Furthermore, inert compounds may be present in the mixture, for examplemethane, ethane, carbon dioxide, nitrogen or argon. Inert compounds maytypically be present in a quantity of from 5 to 80%-mole, more typicallyfrom 10 to 60%-mole, relative to the number of moles of the feed.

The process may preferably be carried out at a temperature in the rangeof from 100 to 250° C., in particular in the range of from 130 to 200°C. As time proceeds, the temperature may be increased gradually, as tocompensate for loss in activity of the catalyst, if any. The process maypreferably be carried out at a pressure in the range of from 1 to 25barg (i.e. bar gauge), in particular in the range of from 1 to 20 barg.

In general, it is preferred to operate at a high oxygen concentration.However, in actual practice in order to remain outside the flammabilitylimits of the reactor streams, the concentration of oxygen has to belowered as the concentration of the olefin and/or oxygenate isincreased. The actual safe operating conditions depend along with thegas composition, also on individual plant conditions, such astemperature and pressure. Therefore, for each individual plant theconcentration of oxygen will be determined which may be used with anyconcentration of the olefin and the oxygenate.

When operating the process as a gas phase process using a packed bedreactor, the GHSV may preferably be in the range of from 1000 to 10000Nl/(l.h). The term “GHSV” stands for the Gas Hourly Space Velocity,which is the volumetric flow rate of the feed, which is herein definedat normal conditions (i.e. 0° C. and 100 kPa (1 bar) absolute), dividedby the volume of the catalyst bed.

The alkenyl carboxylate may be recovered from the reaction product byknown means, such as by fractional distillation or reactivedistillation.

Unless specified otherwise, the organic compounds mentioned herein havetypically at most 10 carbon atoms, in particular at most 6 carbon atoms.Organic compounds are deemed to be compounds which comprise carbon atomsand hydrogen atoms and carbon-hydrogen bonds in their molecules.

It is apparent that certain features of the invention, which are forclarity described herein in the context of separate embodiments, mayalso be provided in combination in a single embodiment. Conversely,features of the invention which are described in the context of a singleembodiment may also be provided separately or in any suitablesub-combination.

The invention will be illustrated by means of the following,non-limiting examples.

EXAMPLE 1 For Comparison

A catalyst was prepared by applying the following steps:

1. A 25 g sample of a silica spheres carrier (spheres diameter 5 mm,surface area 137 m²/g, water absorption capacity 0.63 ml/g, obtainedfrom Südchemie, under the trademark KA-160), was dried at 120° C. inair, cooled down to ambient temperature in a dessicator and subsequentlyimpregnated with 15.7 ml of a solution of sodium palladium (II)tetrachloride (III) (Na₂PdCl₄) and tetrachloroauric acid (HAuCl₄) inde-ionized water containing 0.220 g palladium and 0.121 g gold. Acontainer holding the carrier was gently shaken to allow solution uptakeby the carrier. After complete uptake of the solution the impregnatedcarrier was allowed to stand for 2 hours at room temperature.

2. Then 30 ml of a solution containing 1.68 g sodium metasilicatepentahydrate (Na₂SiO₃.5H₂O) was added to completely cover the wetimpregnated support. This was allowed to stand for 15 hours.

3. Subsequently, 1 ml of 85% w hydrazine hydrate was added, mixed gentlyand allowed to stand for 4 hours at room temperature to reduce thepalladium and gold salts to metallic species.

4. The palladium/carrier composition was then washed with distilledde-ionized water three times by decantation followed by a continuouswash until the wash water was free of chloride, as checked by theabsence of a precipitate when tested with a silver nitrate solution. Thewashed palladium/carrier composition was then dried at about 120° C. for4 hours under nitrogen, and cooled in a container protected frommoisture

5. The palladium/carrier composition was then impregnated with 15.7 mlof a solution of potassium acetate in de-ionized water containing 1.34 gpotassium, dried at about 120° C. for 15 hours under nitrogen, andcooled.

The catalyst thus prepared was tested in a reaction tube having a lengthof 30 cm and an inside diameter of 1.51 cm. The tube was loaded with 2.5g of catalyst diluted into a 10 cm bed of glass beads. The reaction tubewas fed with a gaseous mixture of 49 mole-% ethylene, 13 mole-% aceticacid and 7.6 mole-% oxygen (balance nitrogen). The GHSV was 4250Nl/(l.h), calculated on undiluted catalyst. With the catalysttemperature initially at 147° C., and the absolute pressure at 880 kPa(i.e. 7.8 barg), the space-time-yield was 750 g vinyl acetate/(lcatalyst.h), and the initial selectivity was 93%. The jacket temperaturewas increased slowly so as to keep the space-time-yield constant at 750g vinyl acetate/(l catalyst.h). After 425 hours, the jacket temperaturewas 190° C., and the selectivity was 84%.

EXAMPLE 2 According to the Invention

The procedures of Example 1 was repeated, except that the sample ofsilica spheres carrier was subjected to washing before use in step 1.This washing was carried out by immersing a 500 g sample of the carrierin boiling de-ionized water (6 kg, having a conductivity of 1.5 μmho at98° C.) in a continuously replenishing vessel (flow rate 0.76 l/min).The conductivity of the washing water was measured continuously at 95°C. After 12 minutes the peak value of the conductivity was 60 μmho andafter 120 minutes the conductivity was 6 μmho, at which point thecarrier was subjected to the drying of step 1.

At any point in time the washing water may be deemed to have reachedequilibrium with the carrier. The conductivity data so obtainedrepresent fairly the values which would be obtained when applying theconductivity test, as described hereinbefore, to the unwashed carrierand the washed carrier, respectively.

In testing the catalyst, the space-time-yield was kept constant at 750 gvinyl acetate/(l catalyst.h) by increasing the jacket temperature. Theinitial selectivity was 93%, and after 425 hours, the jacket temperaturewas 168° C., and the selectivity was 91%.

EXAMPLE 3 According to the Invention, Prophetic

A catalyst is prepared in accordance with the procedures of Example 2,except for the following differences: step 3 is omitted, and followingstep 4 the palladium/carrier composition is dried in air at 120° C.,then subjected to reduction in a hydrogen/nitrogen (20:80 v/v) mixture,at a flow rate of the hydrogen/nitrogen mixture of 500 Nl/(lcatalyst.h), at a temperature of 200° C., and at an absolute pressure of110 kPa (1.1 bar), during 4 hours.

EXAMPLE 4 According to the Invention, Prophetic

A catalyst is prepared in accordance with the procedures of Example 2,except for the difference that step 3 is omitted, and that instead thepalladium/carrier composition is dried in air at about 120° C., andcooled, and then subjected to reduction in a hydrogen/nitrogen (20:80v/v) mixture, at a flow rate of the hydrogen/nitrogen mixture of 500Nl/l(l catalyst.h), at a temperature of 200° C., and at an absolutepressure of 110 kPa (1.1 bara), during 4 hours.

EXAMPLE 5

A catalyst was prepared from washed and dried carrier as obtained inExample 2, by applying the following steps:

1. A 25 g sample of the washed and dried carrier was impregnated with15.7 ml of a solution of sodium palladium (II) tetrachloride (III)(Na₂PdCl₄) and tetrachloroauric acid (HAuCl₄) in de-ionized watercontaining 0.220 g palladium and 0.121 g gold. A container holding thecarrier was gently shaken to allow solution uptake by the carrier. Aftercomplete uptake of the solution the impregnated carrier was allowed tostand for 2 hours at room temperature.

2. Then 30 ml of a solution containing 1.68 g sodium metasilicatepentahydrate (Na₂SiO₃.5H₂O) was added to completely cover the wetimpregnated support. This was allowed to stand for 15 hours.

3. Subsequently, 2.5 ml. of 2.8% w hydrazine hydrate in water was added,mixed gently and allowed to stand for 4 hours at room temperature. Thisled to the reduction of about 55 mole-% of the total of the palladiumand gold salts to metallic species.

4. The palladium/carrier composition was then washed with distilledde-ionized water three times by decantation followed by a continuouswash until the wash water was free of chloride, as checked by theabsence of a precipitate when tested with a silver nitrate solution. Thewashed palladium/carrier composition was then dried at about 120° C. for4 hours under nitrogen, and cooled in a container protected frommoisture.

5. The palladium/carrier composition was then subjected to reduction ina hydrogen/nitrogen (15:85 v/v) mixture, at a flow rate of thehydrogen/nitrogen mixture of 500 Nl/(l catalyst.h), at a temperature of220° C., and at a pressure of 100 kPa, until completion of thereduction, i.e. about 2 hours.

6. The palladium/carrier composition was then impregnated with 15.7 mlof a solution of potassium acetate in de-ionized water containing 1.34 gpotassium, dried at about 120° C. for 15 hours under nitrogen, andcooled.

The catalyst thus prepared had a palladium content of 0.75% w, a goldcontent of 0.4% w and its carbon monoxide chemisorption was 25.3 mmolper kg catalyst.

EXAMPLE 6 Prophetic

Vinyl acetate is prepared as follows.

The catalyst prepared in Example 5 is tested in a reaction tube having alength of 30 cm and an inside diameter of 1.51 cm. The tube is loadedwith 2.5 g of catalyst diluted into a 10 cm bed of glass beads. Thereaction tube is fed with a gaseous mixture of 49 mole-% ethylene, 13mole-% acetic acid and 7.6 mole-% oxygen (balance nitrogen). The GHSV is4250 Nl/(l.h), calculated on undiluted catalyst, and the pressure is 880kPa (i.e. 7.8 barg). With the catalyst temperature initially at 147° C.,vinyl acetate is produced. The jacket temperature is increased slowly soas to keep the space-time-yield constant.

EXAMPLE 7 Prophetic

Example 5 is repeated, with the exception that, in step 3, 2.5 ml of 12%w hydrazine hydrate in water is used instead of 2.5 ml of 2.8% whydrazine hydrate in water. This leads in this step 3 to the reductionof about 82-85 mole-% of the total of the palladium and gold salts tometallic species.

The catalyst so prepared is tested in the preparation of vinyl acetateas described in Example 6.

1. A process for preparing a catalyst which process comprises the stepsof (a) precipitating a Group 8 metal compound onto a silica basedcarrier which has been subjected to a series of washings with one ormore aqueous liquids consisting of aqueous liquids which have a pH ofleast 3, when measured at 20° C., or a silica based carrier which isformed from materials one or more of which have been subjected to saidseries of washings; (b) converting the precipitated Group 8 metalcompound into metallic species; and (c) subjecting the Group 8metal/carrier composition resulting from step (a) or step (b) to apurification treatment.
 2. The process as claimed in claim 1, whereinthe washing liquid is water.
 3. The process as claimed in claim 1,wherein the extent of washing is such that in a conductivity test theconductivity measured for the washed carrier is less than 30% of thevalue found for the unwashed carrier, wherein the conductivity testcomprises contacting samples of water with samples of the unwashedcarrier and the washed carrier and measuring at 95° C. the conductivityof each water sample after it has reached equilibrium with therespective carrier sample at 95° C., the quantity of water sample being3 g/g carrier sample, the conductivity of the water prior to thecontacting with a carrier sample being 1.5 μmho at 98° C., and theconductivities being measured by using a conductivity measuring probehaving a cell constant of 1.0/cm.
 4. The process as claimed in claim 3,wherein the extent of washing is such that in the conductivity test theconductivity measured for the washed carrier is less than 20% of thevalue found for the unwashed carrier.
 5. The process as claimed in claim1, wherein the extent of washing is such that in a conductivity test theconductivity measured for the washed carrier is less than 75 μmho,wherein the conductivity test comprises contacting a sample of waterwith a sample of the washed carrier and measuring at 95° C. theconductivity of the water sample after it has reached equilibrium withthe carrier sample at 95° C., the quantity of water sample being 3 g/gcarrier sample, the conductivity of the water prior to the contactingwith the carrier sample being 1.5 μmho at 98° C., and the conductivitiesbeing measured using a conductivity measuring probe having a cellconstant of 1.0/cm.
 6. The process as claimed in claim 5, wherein theextent of washing is such that in the conductivity test the conductivitymeasured for the washed carrier is less than 50 μmho.
 7. The process asclaimed in claim 1, wherein the Group 8 metal and optionally in additiona Group 1b metal is precipitated onto the carrier by pore impregnatingthe carrier with one or more aqueous solutions comprising a Group 8metal compound precursor and optionally a Group 1b metal compoundprecursor and then precipitating a Group 8 metal compound and optionallya Group 1b metal compound onto the carrier from such solutions, by usinga precipitating agent.
 8. The process as claimed in claim 7, wherein theGroup 8 metal compound precursors and the optional Group 1b metalcompound precursors are selected from water soluble acids and salts ofpalladium and optionally gold and the precipitating agent is selectedfrom the group consisting of hydroxides, bicarbonates, carbonates andsilicates of lithium, sodium and potassium.
 9. The process as claimed inclaim 1, wherein the precipitated Group 8 metal compound and, ifpresent, a precipitated Group 1b metal compound is converted intometallic species by reduction employing hydrogen as the reducing agent.10. The process as claimed in claim 1, wherein the purificationtreatment involves a series of washings with one or more aqueousliquids.
 11. The process as claimed in claim 1, which comprises inaddition a step of (d) impregnating the purified Group 8 metal/carriercomposition with a source of an alkali metal.
 12. A catalyst prepared bya process comprising the steps of (a) precipitating a Group 8 metalcompound onto a silica based carrier which has been subjected to aseries of washings with one or more aqueous liquids consisting ofaqueous liquids which have a pH of least 3, when measured at 20° C., ora silica based carrier which is formed from materials one or more ofwhich have been subjected to said series of washings; (b) converting theprecipitated Group 8 metal compound into metallic species; and (c)subjecting the Group 8 metal/carrier composition resulting from step (a)or step (b) to a purification treatment.
 13. The catalyst as claimed inclaim 12, which comprises palladium as the Group 8 metal in a quantityin the range of from 10 mmoles/kg to 500 mmoles/kg catalyst, and inaddition gold as a Group 1b metal in a quantity in the range of from 1mmoles/kg to 200 mmoles/kg catalyst, and an alkali metal in a quantityin the range of from 0.1 mmoles/kg to 5 mole/kg catalyst.
 14. A processfor preparing an alkenyl carboxylate comprising reacting a mixturecomprising an olefin, a carboxylic acid and oxygen in the presence of acatalyst prepared by a process comprising the steps of (a) precipitatinga Group 8 metal compound onto a silica based carrier which has beensubjected to a series of washings with one or more aqueous liquidsconsisting of aqueous liquids which have a pH of least 3, when measuredat 20° C., or a silica based carrier which is formed from materials oneormore of which have been subjected to said series of washings; (b)converting the precipitated Group 8 metal compound into metallicspecies; and (c) subjecting the Group 8 metal/carrier compositionresulting from step (a) or step (b) to a purification treatment.
 15. Theprocess as claimed in claim 14, wherein the catalyst comprises palladiumas the Group 8 metal in a quantity in the range of from 10 mmoles/kg to500 mmoles/kg catalyst, and in addition gold as a Group 1b metal in aquantity in the range of from 1 mmoles/kg to 200 mmoles/kg catalyst, andan alkali metal in a quantity in the range of from 0.1 mmoles/kg to 5mole/kg catalyst.